A design method of a hypoid gear is described in Ernest Wildhaber, Basic Relationship of Hypoid Gears, American Machinist, USA, Feb. 14, 1946, p. 108-111 and in Ernest Wildhaber, Basic Relationship of Hypoid Gears II, American Machinist, USA, Feb. 28, 1946, p. 131-134. In these references, a system of eight equations is set and solved (for cone specifications that contact each other) by setting a spiral angle of a pinion and an equation of a radius of curvature of a tooth trace, in order to solve seven equations with nine variables which are obtained by setting, as design conditions, a shaft angle, an offset, a number of teeth, and a ring gear radius. Because of this, the cone specifications such as the pitch cone angle Γgw depend on the radius of curvature of the tooth trace, and cannot be arbitrarily determined.
In addition, in the theory of gears in the related art, a tooth trace is defined as “an intersection between a tooth surface and a pitch surface”. However, in the theory of the related art, there is no common geometric definition of a pitch surface for all kinds of gears. Therefore, there is no common definition of the tooth trace and of contact ratio of the tooth trace for various gears from cylindrical gears to hypoid gears. In particular, in gears other than the cylindrical gear and a bevel gear, the tooth trace is not clear.
In the related art, the contact ratio mf of tooth trace is defined by the following equation for all gears.mf=F tan ψ0/p where, p represents the circular pitch, F represents an effective face width, and ψ0 represents a spiral angle.
Table 1 shows an example calculation of a hypoid gear according to the Gleason method. As shown in this example, in the Gleason design method, the tooth trace contact ratios are equal for a drive-side tooth surface and for a coast-side tooth surface. This can be expected because of the calculation of the spiral angle ψ0 as a virtual spiral bevel gear with ψ0=(ψpw+ψgw)/2 (refer to FIG. 9).
The present inventors, on the other hand, proposed in Japanese Patent No. 3484879 a method for uniformly describing the tooth surface of a pair of gears. In other word, a method for describing a tooth surface has been shown which can uniformly be used in various situations from a pair of gears having parallel axes, which is the most widely used configuration, to a pair of gears whose axes do not intersect and are not parallel with each other (skew position).
There is a desire to determine the cone specifications independent from the radius of curvature of the tooth trace, and to increase the degree of freedom of the design.
In addition, in a hypoid gear, the contact ratio and the transmission error based on the calculation method of the related art are not necessarily correlated to each other. Of the contact ratios of the related art, the tooth trace contact ratio has the same value between the drive-side and the coast-side, and thus the theoretical basis is brought into question.
An advantage of the present invention is that a hypoid gear design method is provided which uses the uniform describing method of the tooth surface described in JP 3484879, and which has a high degree of freedom of design.
Another advantage of the present invention is that a hypoid gear design method is provided in which a design reference body of revolution (pitch surface) which can be applied to the hypoid gear, the tooth trace, and the tooth trace contact ratio are newly defined using the uniform describing method of the tooth surface described in JP 3484879, and the newly defined tooth trace contact ratio is set as a design index.